1. Technical Field of the Invention
The present invention relates to a malfunction diagnosing apparatus for an internal combustion engine which enables selection of a fuel to be supplied to a fuel tank from a plurality of fuels having different properties.
2. Related Art
Recently, there is a trend of socially seeking decrease in the amount of emission of CO2 or seeking utilization of fuels that can replace oil. In response to this, there has been an increasing demand for vehicles installing an engine (internal combustion engine) that can use any of gasoline, alcohol and a mixture of the both as a fuel. In such a vehicle, a fuel having an alcohol concentration different from that of the previously supplied fuel may often be supplied to the fuel tank. With the supply of such a fuel of different alcohol concentration, the alcohol concentration of the fuel in the fuel tank will change. Since the theoretical air-fuel ratio is different between gasoline and alcohol, the change in the alcohol concentration of the fuel will accompany the change in the theoretical air-fuel ratio of the fuel. Therefore, it is required that the fuel injection quantity (actual air-fuel ratio) be changed according to the alcohol concentration of the fuel.
A countermeasure against this issue is suggested in JP-A-H03-085347. In the apparatus disclosed in JP-A-H03-085347, an alcohol concentration sensor is provided in the fuel path extending from the fuel tank to the fuel injection valves to sense alcohol concentration of the fuel. According to the alcohol concentration sensed by the alcohol concentration sensor, a correction amount for the current alcohol concentration (hereinafter referred to as “alcohol concentration correction amount”) is determined for the fuel injection quantity. At the same time, under air-fuel ratio feedback control, a correction amount for air fuel ratio feedback (hereinafter referred to as “air-fuel ratio feedback correction amount”) is determined based on the difference between the target air-fuel ratio and the actual air-fuel ratio. Thus, the apparatus is configured to correct the fuel injection quantity using the alcohol concentration correction amount and the air-fuel ratio feedback correction amount.
Further, recent engine control systems using advanced electronic control are disclosed in, for example, JP-A-2008-038785 and JP-B-H07-006430. Such an engine control system includes a self-diagnostic function that performs various malfunction diagnoses such as for a fuel system and for an alcohol concentration sensor.
If malfunction occurs in a fuel system, the fuel necessary for the required injection quantity may no longer be injected. For this reason, the difference between the target air-fuel ratio and the actual air-fuel ratio becomes excessively large, and thus the air-fuel ratio feedback correction amount will result in an abnormal value exceeding a predetermined range. Also, if malfunction occurs in an alcohol concentration sensor, the alcohol concentration of the fuel may be erroneously sensed. Accordingly, the alcohol concentration correction amount for the fuel injection quantity will result in an abnormal value. For this reason, the difference between the target air-fuel ratio and the actual air-fuel ratio becomes excessively large, and thus the air-fuel ratio feedback correction amount will result in an abnormal value.
As will be understood from the relationship mentioned above, malfunction diagnosis of a fuel system and malfunction diagnosis of an alcohol concentration sensor are both ensured to be conducted using the air-fuel ratio feedback correction amount. However, in the case where malfunction diagnosis of a fuel system and malfunction diagnosis of an alcohol concentration sensor are conducted in combination, no definitive determination can be made at all whether the abnormal value of the air-fuel ratio feedback correction amount indicates malfunction in the fuel system or malfunction in the alcohol concentration sensor.